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Microsoft word - pavelkaabstr-vienna091.doc

Radiopharmacy Adaptation of methods for radiometric determination of enzyme activities of the key enzymes of thyroid hormones metabolism

Stanislav Pavelka a,b* aInstitute of Physiology, Academy of Sciences of the Czech Republic, v.v.i., 14220 Prague 4, Czech Republic; bDepartment of Biochemistry, Faculty of Science, Masaryk University, 61137 Brno, Czech Republic * Corresponding author: pavelka@biomed.cas.cz or pav@chemi.muni.cz The aims of the present studies were: first, to establish valid
high bromide intake its thyrotoxic effects prevailed and TPO
assay conditions for the radiometric determination of the activity was reduced. enzyme activities of thyroid peroxidase (TPO), iodothyronine
Interestingly, elevated TPO activity was found in all rats
sulfotransferases (ST) and uridinediphospho-glucuronyl-
administered with perchlorate, regardless of the type of diet
transferase (UDPGT) in various fractions of rat tissues; and
(the content of iodine in the diet, respectively).
second, with the aid of such developed assays, to follow the
effects of some exogenous substances - bromide and
perchlorate ions as supposed goitrogenic agents, and an
antidepressant drug fluoxetine (Prozac) - on the metabolism
The mentioned enzymes are involved either in the
biosynthesis of TH in the thyroid gland (TPO), or in
peripheral biotransformations of TH (ST and UDPGT, along
with the most important iodothyronine deiodinases).
Br conc. (g/l)Br conc. (g/l)
Fig. 1 Specific peroxidase (TPO) activity, determined in microsomal
Methods
fractions of the thyroid glands of rats maintained for up to 56 days on the iodine-sufficient diet B or iodine-deficient diet R, in dependence
The procedure of the radiometric assay for TPO in vitro was
on the extent of bromide intake. (The rats permanently drank
based on the ability of TPO to oxidize 131I-iodide in the
solutions of bromide with the concentrations of 0, 1, 2, 3 or 5 g/l).
2O2, generated in situ by glucose oxidase, and
to catalyze subsequent iodination of specific tyrosyl residues
The radiometric determination of iodothyronine sulfo-
in the added thyroglobulin. The measure of TPO activity in
transferases (ST) did not demonstrate any significant effects
microsomal fractions of the thyroids was the amount of of the application to the rats of fluoxetine alone, or together radioiodine incorporated into thyroglobulin, determined with T3, on the induction of these enzyme activities. either after TLC separation of the incubated samples, or
In contrast, administration of fluoxetine alone caused a
simply after precipitation of radiolabeled protein and significant (about 2-fold) increase in glucuronyl-transferase measurement of 131I radioactivity in separated fractions.
(UDPGT) activity (Table 1). However, the effect of
The transfer of sulfonate moiety from the "active fluoxetine was completely abolished, provided it was applied
sulfate", 3'-phosphoadenosine 5'-phosphosulfate, to the in combination with supraphysiological concentrations of T3. substrate L-3,3',5'-[125I]-triiodothyronine ([125I]-rT
basis for radiometric determination of ST activity in liver
cytosolic fractions. The extent of conversion was determined
after the separation of the sulfated product from the
unmodified [125I]-rT3 by Sephadex LH-20 chromatography.
The rate of conjugation of the phenolic hydroxyl group
3 with glucuronic acid, catalyzed by UDPGT in rat
liver microsomes, was measured in the radiometric assay for
UDPGT. Reaction mixtures, containing uridinediphospho-glucuronic acid, the substrate [125I]-rT
Table 1: The influence of fluoxetine, 3,5,3’-triiodothyronine (T3) and
microsomes, were analyzed after the incubation, again by
their combination on specific enzyme activities (fmol rT3-G/h/mg prot.) of UDPGT in rat liver microsomes.
chromatography on microcolumns of Sephadex LH-20.
Results and Discussion Acknowledgements: This work was supported by the Academy of Sciences of the Czech Republic (Research project No. AV0Z
With the use of the described radiometric assay for TPO, we
50110509), by the Ministry of Education of the Czech Republic
found that the influence of exogenous bromide on the TPO
(Research project No. MSM0021622413), by the GA CR (Grant No.
activity in the rat thyroids was biphasic, with regard to the
304/08/0256), GA AS CR (Grant No. KJB401630701) and by the
extent of bromide intake in the animals (Fig. 1). An increase
grant from the EC (Project MYORES, contract No. 511978).
(up to 3-fold) in TPO activity was measured in rats with a low or moderate bromide intake while in animals with very